• Background: We evaluated the effects of Hartmann's lactated Ringer's (HLR) solution, balanced salt solution (BSS) and BSS Plus on human corneal endothelium. • Methods: Paired human corneas were mounted in the in vitro specular microscope for endothelial perfusion with HLR, BSS or BSS Plus for 15, 30, 60 and 120 min. Reversal experiments with BSS Plus after initial HLR perfusion were performed. At the end of the perfusions, electron microscopy, F-actin staining of the endothelial cytoskeleton and endothelial permeability measurements were carried out. • Results: Longterm perfusion (120 min) with HLR resulted in a significantly higher swelling rate than in the paired controls perfused with BSS Plus. Short-term exposure to HLR for 15, 30 and 60 min after initial BSS Plus perfusion increased the swelling rates significantly. The increased corneal swelling after HLR perfusion for 60 min was reversed by BSS Plus perfusion. Ultrastructural changes in HLR-perfused corneas included endothelial cell edema, cytoplasmic vacuolation and mitochondrial swelling. F-actin staining showed overall cytoskeletal disorganization after perfusion with HLR. Corneal endothelial permeability was higher for BSS Plus-perfused corneas than with HLR solution. • Conclusion: The results suggest that the clinically observed corneal clouding during irrigation with HLR is due to endothelial cell edema and decreased endothelial pump function. However, this increased corneal swelling is reversible by perfusion with BSS Plus.